1. Field of the Invention
The present invention relates to bandwidth compression and expansion systems, and more particularly to systems for processing signals such as audio and video signals which can be bandwidth compressed for transmission or other processing followed by decompression or other expansion.
2. History of the Prior Art
It is well known in the transmission of certain signals such as video and audio signals to compress the signal bandwidth for purposes of such transmission. At the receiving end, the transmitted signals are subjected to a decompression process to produce a signal which is a close approximation of the original signal transmitted.
A number of different compression and expansion techniques are commonly employed in connection with the transmission or other processing of certain signals. In the case of audio and video signals, for example, the human ear and the human eye cannot perceive certain differences or changes in such signals. Therefor, it is often possible to selectively sample the signal information and to transmit only the sampled information to the exclusion of the other information contained within the signal. Interpolation or extrapolation techniques may then be used to produce an audio or video signal which is a reasonably close approximation of the original audio or video signal transmitted. In most instances the human ear and the human eye cannot detect differences or aberrations in the decompressed signal.
Examples of conventional systems for processing audio and video signals and the like are provided by several publications which include pages 77-82 of Digital Image Processing by William B. Green, pages 238-243 of Digital Filters and Signal Processing by Leland B. Jackson, and pages 320-323 and 494-497 of Digital Coding of Waveforms by Jayant and Noll.
Conventional bandwidth compression and expansion systems for the processing of video signals typically sample only certain pixels to the exclusion of all others and then rely upon interpolation of extrapolation to reconstruct the complete video signal at the receiving end. Typically, only every third or fourth pixel of the video image is sampled and stored, with the result that as much as 75% or more of the information contained within such signal is thereby discarded or thrown away. This occasionally results in variations or aberrations in the transmitted signal which are noticeable to the viewer. It is particularly troublesome in the case of certain shapes or patterns within the picture that cannot be accurately reproduced where the intervening pixel information between the sampled data is missing. Still further problems arise because of the inability of such conventional systems to process the information in real time. Typically, the two different fields of each picture frame are examined and sampled, with as many as eight fields being examined because of the rapidly occurring frames and the time required to accomplish such processing. As a result, temporal filtering sometimes occurs in which signal processing takes place within a void between samples so that the moving picture is lost. Still other problems occur in areas such as high definition television where a larger than normal amount of picture information must be transmitted or otherwise processed in a manner so that the enhanced picture information is not lost.
It would therefore be advantageous to be able to process signals such as video signals in real time. This would enable each frame of a video signal or other visual signal, for example, to be compressed, expanded or otherwise processed as the frame occurs or is presented, thereby eliminating the need for simultaneous processing of the fields of different images with the consequent loss of motion or other problems which sometimes occur. It would also be advantageous to provide a compression scheme or similar signal processing scheme in which all of the pixels or other signal values of the signal being processed are examined and used in generating the compressed signal, rather than large portions of such signal information simply being discarded. It would still further be advantageous to provide signal expansion apparatus which can be used for image enhancement such as in the case of a synthesizer, in which the aspect ratio of an image can be changed where desired.